This invention relates generally to a multiple beam raster output scanning system and, in particular, to bow compensation in multiple reflection multiple beam raster output scanning systems. Multiple reflection multiple beam raster output scanning systems use multiple reflections between the facet of the rotating polygon mirror and a second mirror for passive facet tracking and scan angle doubling.
Prior art FIG. 1 shows a known rotating polygon multi-beam raster outputs scanner (ROS). The functions described below equally apply to most rotating polygon scanning systems, independently of number of light sources used.
FIG. 1 shows a pair of sagittally offset laser diodes 31 and 32. The beams 41 and 42 emitted by laser diodes 31 and 32 are collimated by the collimator 33 (lens L1). A sagittal aperture 34 is placed in the position where the beams 41 and 42 cross the optical axis, to control the F/#, which in turn controls the spot size. The input cylinder optical element 35 (lens L2) focuses the beams 41 and 42 on the surface of the current polygon facet 36 of the rotating polygon. After reflecting from the current facet 36, the beams 41 and 42 pass through the F-theta lens 37 (lens L3). The F-theta lens 37, in general, has relatively low power in the tangential meridian. The main function of the F-theta lens 37 is to provide focusing in the tangential meridian and control the scan linearity, in terms of uniform spot displacement per unit angle of polygon rotation.
The function of the motion compensating optical element (MCO) 39 is to reimage the focused beams 41 and 42 from the polygon facet 36 onto the Photoreceptor (PR) plane 40 at a predetermined position, independently of the polygon angle error or tilt of the current facet 36. Such compensation is possible because the focused beams are stationary "objects" for the F-theta lens 37 and the MCO 39. Although, due to polygon tilt, or wobble, the beams 41 and 42 are reflected to different positions of the post-polygon optics aperture for each different facet of the rotating polygon, the beams 41 and 42 are imaged to the same position on the PR plane 40.
In rotating polygon, single spot ROS-based xerographic copiers and printers, bow distortions occur from the accumulation of optics tolerances. Bow itself is the curved line described by the scanned laser beam of the ROS as it moves in the fast scan direction. Thus, the bow appears as a displacement of a scan line in the process direction as the line extends in the fast scan direction.
Although multi-beam, laser diode based ROS provides high quality, high throughput xerographic printing, differential scan line bow remains as an undesirable side affect. Differential scan line bow arises from the very nature of multi-beam optical systems, where the beams are offset sagittally (in the cross-scan direction) so that half of the beams lie above and half of the beams lie below, or all of the beams lie above or below, the optical axis.
Depending on the design of the system, the differential scan line bow can cause the scan lines to move toward each other (barrel distortion), or away from each other (pin cushion distortion). In both of these cases, the light sources (lasers) are placed on opposite sides of the optical axis. Therefore, the centers of curvature of the bowed scan lines are also on opposite sides of the optical axis. If all light sources are placed on one side of the optical axis, then all of the scan lines will be imaged on the opposite side of the optical axis. Therefore, the centers of curvature of all of the bowed lines will also lie on same side of the axis. However, each line will be bowed at a different radius of curvature. This is the source of another type of differential bow.
FIGS. 2 to 5 show the various types of errors which can be introduced by differential scan line bow. In FIG. 2, the ideal scan line 50 is shown as a dashed lines. The first bowed scan line 53 has a first radius of curvature which is different from the radius of curvature of the second bowed scan line 54.
In FIG. 3, a third bowed scan line 56 is superimposed over the first bowed scan line 52. As shown in FIG. 3, the third bowed scan line 56 has a center of curvature which is on the opposite side of the ideal scan line 50 from the center of curvature of the first bowed scan line 52.
In FIG. 4, the center of curvatures of bowed scan lines 51 and 53 are located on the opposite side of ideal scan line 50 in such a fashion that the bowed scan lines create a pincushion distortion. This occurs whether the bowed scan lines 51 and 53 have the same or different radius of curvature.
In FIG. 5, the center of curvatures of bowed scan lines 55 and 57 are also on the opposite side of ideal scan line 50 (with same or different radii) but their arrangement with relation to each other is such that they form a pincushion distortion. Again, this occurs whether the bowed scan lines 55 and 57 have the same or different radii of curvature.
In general, in the first order, all of these distortions in the prior art systems are generated by the considerable angular deviation between the output chief rays and the system axis, as shown in FIG. 1.
In single-beam monochrome or single-beam multiple pass color printing systems, a few hundred microns of bow causes no noticeable degradation in the image quality because the bow of the successive scan lines is identical. However, in multiple beam, monochrome, single-station printing systems, or in multiple beam, single-pass color printing systems with single or multiple photoreceptor stations, differential bow causes gross misregistration on the photoreceptor(s) both in the single monochrome image and also among the color layers in the multi-layer color image.
In particular, this misregistration can occur because the magnitude and the earlier described different orientation of the differential bow.
Multiple reflection ROS use multiple reflections between the facet of the rotating polygon mirror and a second mirror for passive facet tracking and scan angle doubling.
As taught in U.S. Pat. No. 5,475,524, commonly assigned as the present application and herein incorporated by reference, scanner performance is determined by the physical limitations on the speed at which the polygon mirror is rotated, by the angular deflection of the laser beam achieved by reflection from a facet from the rotating polygon, the number of facets, the size of the facets, and the width of the beam being scanned where it is incident upon the rotating polygon mirror.
The beam width impacts the scanning speed because it determines the minimum facet size of a facet on the rotating polygon mirror. A larger facet means a larger rotating polygon and hence larger, more costly, motor polygon assemblies with higher power motors and slower scanning speeds. Scanning speeds, for a given beam width, can be increased by the use of facet tracking devices because they allow a smaller facet to be used and therefore smaller rotating polygon mirrors which can be rotated faster.
One method for increasing scanning speeds is the use of angle doubling with small sized polygon assemblies having a large number of small sized facets. For an F-theta scan lens, commonly employed in laser ROS, the scanned distance on the photoreceptor is the product of the scan angle (theta) and the effective focal length (f). Whenever the scan angle can be increased, the effective focal length can be decreased for a given scan length. A decrease in the effective focal length brings two primary advantages. Firstly, the smaller focal length translates directly into a smaller physical casting or base upon which the optical components are mounted. Glass lens elements, mirrors and all other components can be smaller. The end result is a smaller, lighter, less costly product. Secondly, the shorter focal length requires a smaller beam at the rotating polygon mirror, further reducing the sizes of the optical and mechanical components.
A further advantage results from scan angle doubling in that any given scan distance along the photoreceptor can be achieved with only half the polygon angular rotation. By this means, the polygon speed of rotation is significantly reduced, allowing lighter, smaller and less costly motor bearings as well as better bearing lifetime and overall performance.
To be free from differential scan line bow, a laser beam approaching the photoreceptor plane can have no cross-scan component to its propagation direction vector. This condition is easily achieved for a single beam ROS. One method for elimination differential scan line bow in a multiple beam ROS is to make the multiple beams telecentric, i.e. parallel to the optical system axis, as taught in U.S. Pat. No. 5,512,949, commonly assigned as the present application and herein incorporated by reference.
However, in a multiple beam, multiple reflection ROS for facet tracking and scan angle doubling, providing telecentric beams alone is not sufficient to compensate for differential scan line bow.
It is an object of the present invention to provide optical means to compensate for differential scan line bow in a multiple beam, multiple reflection ROS for facet tracking and scan angle doubling.